The brewing of beer is an ancient art. Malting and fermentation of cereals to produce "beer" commenced between about 2000 B.C. and 3000 B.C. in Mesopotamia. The art of brewing beer in Europe has been practiced since at least the Fourth Century and most likely earlier. However, the basic ingredients for brewing beer have remained essentially unchanged over the centuries: barley, malt, water, hops and yeast. The Beer Purity Laws enacted in Bavaria in 1516--the Reinheitsgebot--limit brewing materials to these ingredients and are still followed today in all of Germany with respect to domestically consumed beer.
The basic process for brewing beer has not significantly changed over the centuries and essentially includes:
malting barley by germination--the purpose of malting is to bring forth enzymes that break down starch and proteins to less complex water soluble compounds, i.e. amino acid, fermentable sugars and small peptides;
crushing malted barley to create "grist";
adding water to the grist to create a mash;
separating the resulting aqueous extract, known as the "wort" which is rich in fermentable sugars and other nutrients;
boiling the wort with hops to add flavors and stop enzymatic action;
cooling and clarifying the wort;
fermenting the wort with yeast to convert glucose to ethanol and carbon dioxide ("primary fermentation") to produce the "green" or "young" beer;
maturing or "lagering" the fermented "green" or "young" beer, generally by means of "secondary fermentation", again using yeast;
filtering, pasteurizing and packaging the beer.
The maturation or "lagering" step is an exceptionally important aspect of the process; careful and exact lagering is required to create beer with mature and consistent flavor.
During primary fermentation, one of the first phases is yeast growth. During this phase--as the yeast cells multiply --all biochemical activities including amino acid synthesis are "switched on". In the biochemical pathway of valine synthesis (an amino acid), alpha acetolactate is formed in excess quantities and secreted by the yeast cells into the wort. In the wort outside the yeast cells the alpha acetolactate is converted chemically (not enzymatically) by decarboxylation to a diketone, diacetyl (CH.sub.3 COCOCH.sub.3); the conversion of alpha acetolactate to diacetyl is a chemical reaction controlled by the pH, temperature and the redox state of the beer.
Many diketones, including diacetyl, are strong flavor components and have very low taste threshold; diacetyl can be detected at concentrations of about 0.05 ppm and has a strong buttery or butterscotch flavor, which is a characteristic of "green" or "young" or immature beer. In order to balance the flavor of beer after primary fermentation, lagering or other forms of maturation are utilized to reduce the concentration of diacetyl below that of the human taste threshold. The classic method of lagering involves "secondary fermentation" wherein the green beer is fermented for a second time by yeast; typically, the beer from the primary fermentation tank is run into an enclosed lager tank with yeast at about 2-5.times.10.sup.6 per/ml and fermentable sugars. The yeast cells--during this phase--are transporting diacetyl into the cells where the diacetyl is enzymatically reduced by diacetyl reductase or other enzymes to yield acetoin; acetoin has a much less intense flavor and a significantly higher taste threshold, i.e. about 50 ppm-1,000 ppm, than diacetyl.
The reaction rate of diacetyl to acetoin is much faster than the reaction rate of acetolactale to diacetyl under standard brewing temperatures and conditions. However, in order to prevent the formation of diacetyl in the maturation stage, it is necessary to limit the amount of diacetyl precursors (including alpha acetolactate) present as the beer enters the maturation stage. If this is not accomplished, "potential" diacetyl remains which, during or after the maturation stage, can alter the flavor of the beer.
During primary fermentation, diketones other than diacetyl are also produced and the maturation process adjusts other flavor elements. However, the primary goal of maturation--and the most understood--is the reduction of diacetyl concentration.
Secondary fermentation is, however, a lengthy process, which can typically last two weeks or more and is always conducted as a "batch" process. Commercial brewers--who brew beer in vast quantities--have sought methods which "mature" beer faster, cheaper and more efficiently. Among the prior art methods utilized is "ageing" the beer, i.e. fermenting the beer to completion during primary fermentation so that very little diacetyl remains; the beer is then held at 2.degree. C.-4.degree. C. for two to four days. This can be accomplished in a single, universal tank wherein the beer is fermented, aged and finished in a single tank without any transfers. This "ageing" process is practiced by most major breweries in the United States.
Another process involves a diacetyl "rest". In this process, the green beer is held at a temperature of 12.degree. C.-18.degree. C. for a week following primary fermentation, followed by secondary fermentation at lower temperatures. This "rest" allows all or substantially all of the alpha acetolactate to be secreted by the yeast and reduced to diacetyl, which is subsequently enzymatically reduced to acetoin during maturation.
However, these methods still require substantial time to properly "age" or "mature" the beer. Neither of these methods --or any other prior art methods--are "continuous" processes; utilization of a continuous maturation process could potentially increase the efficiency of and reduce the brewing time and cost for the production of beer. The instant invention, however, substantially reduces the amount of maturation time and provides--for the first time--a continuous, efficient and relatively inexpensive method for the maturation of fermented beer.